My invention relates to electrically resistive bodies composed primarily of metallic oxides and to a method of fabricating such resistive bodies. The resistive bodies according to my invention are perhaps best suited for use as varistors by reason of their markedly nonlinear volt-ampere characteristic, although I do not wish my invention to be limited to this particular application.
The varistor is a two-electrode semiconductor device, sometimes referred to as voltage-dependent resistor because of its voltage-dependent nonlinear resistance. It has found extensive use in electronic circuits for the absorption of voltage surges.
I know some semiconductor materials heretofore suggested and used for varistors. For example, Japanese Patent Publication No. 53-11076 proposes the sintered moldings of zinc oxide (ZnO), bismuth trioxide (Bi.sub.2 O.sub.3), cobaltous oxide (CoO), manganous oxide (MnO), antimony trioxide (Sb.sub.2 O.sub.3), nickel oxide (NiO) and silica (SiO.sub.2). Japanese Unexamined Patent Publication No. 61-43404 discloses varistor compositions explicitly designed for higher antisurge capabilities, comprising ZnO; Bi.sub.2 O.sub.3 ; one or more of CoO, MnO, magnesium oxide (MgO), calcium oxide (CaO), strontium oxide (SrO), barium oxide (BaO), NiO, SiO.sub.2, tin dioxide (SnO.sub.2), titanium dioxide (TiO.sub.2), germanium dioxide (GeO.sub.2), Sb.sub.2 O.sub.3, boric oxide (B.sub.2 O.sub.3) and chromic oxide (Cr.sub.2 O.sub.3); one or more of ytterbium oxide (Yb.sub.2 O.sub.3), erbium oxide (Er.sub.2 O.sub.3), yttrium oxide (Y.sub.2 O.sub.3), lanthanum oxide (La.sub.2 O.sub.3), praseodymium oxide (Pr.sub.2 O.sub.3) and neodymium oxide (Nd.sub.2 O.sub.3); aluminum oxide (Al.sub.2 O.sub.3); and lithium oxide (Li.sub. 2 O).
Notwithstanding such known compositions, there have been consistent demands from the electronics and allied industries for varistor materials capable of protecting electronic appliances against higher voltage surges. There have also been demands for varistor materials having a higher volt-ampere nonlinearity coefficient in a wide range of varistor current magnitudes.